There are demands for endoscopes that allows the user to perform general observation of a wide range and detailed local observation of an affected area, or the like, which is found during the general observation. Conventionally, such demands have often been met by using a fixed focus lens with deep depth of field. However, further improvement of image quality is now desired, since there are trends of endoscopes with higher pixel density and wider angle of view, or analysis or observation is performed using images captured with an endoscope.
Considering the above-described situation, endoscope objective lenses that can be changed between a far point side observation state (WIDE) suitable for general observation and a near point magnified observation state (TELE) suitable for local observation depending on the situation of use of the endoscope are now being used. As prior art examples thereof, objective lenses disclosed in Japanese Patent No. 2876252 and Japanese Unexamined Patent Publication Nos. 2001-091832 and 2009-294496 (hereinafter, Patent Documents 1, 2 and 3, respectively) for example, are known.
Patent Document 1 discloses an objective lens having a four-group configuration with a negative-positive-negative-positive refractive power arrangement, wherein the above-described change of the state is achieved by moving only the third group. Patent Document 2 discloses an objective lens having a four-group configuration with a negative-positive-negative-positive refractive power arrangement, wherein the above-described change of the state is achieved by moving the third lens group and one of the second and fourth lens groups. Patent Document 3 discloses an objective lens having a three-group configuration with a positive-negative-positive refractive power arrangement, wherein the above-described change of the state is achieved by moving only the second group.